In the method for manufacturing an alloy ingot having a round-rod shape through hot forging, the alloy ingot is heated and subjected to a forging process, and the forging process is finished before a temperature of the alloy ingot is decreased to a predetermined temperature, or the alloy ingot is reheated and repeatedly subjected to the forging process. In consideration of efficiency in the forging process, a predetermined amount of forging processing can be desirably completed by one-time heating of the alloy ingot rather than performing reheating. In this regard, a forging method in which a temperature decrease of the alloy ingot is minimized to secure a long processing time has been proposed.
For example, Patent Document 1 discloses a forging method for performing hot forging while minimizing a temperature decrease by coating an alloy ingot (a workpiece) such as an ultra heat-resistant alloy with a heat-resistant ceramic fibrous material. First, a heat-retaining sheet Ruined of a heat-resistant ceramic fibrous material is prepared and thereby is covered an outer peripheral surface of the alloy ingot. Furthermore, the heat-retaining sheet is fixed by using a stainless steel foil and a stainless steel band. Then, the resulting one is heated, and is subjected to high-speed tetrahedral forging such that a plurality of passes of a forging process are performed by one-time heating. As compared with a case where the outer peripheral surface is not coated with the heat-retaining sheet, the decreasing rate of the temperature of the alloy ingot can be made slow due to a heat retaining effect of the heat-retaining sheet, and thus a long processing time can be taken until the temperature decreases to a predetermined temperature in one-time heating, and thereby a large amount of the forging processing can be obtained. In addition, if the heat-retaining sheet is deliberately adjusted in advance such that it easily becomes damaged, it is possible to take the heat-retaining sheet off by just bringing a processing peripheral blade into contact with the heat-retaining sheet to drop it down without interfering with a finishing surface.
However, in the case of a high-alloy which has relatively high deformation resistance at the time of forging, such as an ultra heat-resistant alloy, as also disclosed in Patent Document 1, cracks are easily generated due to a temperature decrease during a forging process. The cracks generated due to the temperature decrease during the forging process of the hard-to-work alloy are easily generated not only in an alloy having relatively high deformation resistance, but also in an alloy such as an age-hardening alloy in which a precipitated phase appears at a certain temperature or lower to rapidly increase the deformation resistance. In a forging process of the aforementioned alloys, it is necessary to strictly control the forging temperature to be a predetermined temperature or higher at all times; however, in a method of simply winding the heat-retaining sheet around the alloy ingot as disclosed in Patent Document 1, since the followability of the heat-retaining sheet for the deformation of the alloy ingot is not sufficient, a gap is generated between the alloy ingot and the heat-retaining sheet or the heat-retaining sheet drops down during the forging process, and thereby it is not possible to stably retain the heat of the alloy ingot in some cases. In this regard, a method in which an alloy ingot is fitted into a tube and then subjected to a forging process has been proposed, that is, a method in which a heat-retaining member formed of metal coating is provided around an alloy ingot and the resulting one is subjected to a forging process has been proposed.
For example, Patent Document 2 discloses a “insert-casting” method in which a round-rod shaped alloy ingot formed of an age hardening Ni-based ultra heat-resistant alloy is inserted into a mold so as to stand upright on a bottom portion while not being in contact with the inner peripheral surface of the mold, and heat-retaining molten metal is poured into a gap between the alloy ingot and the mold, to thereby “insert-casting” the alloy ingot by a heat-retaining metal member. The alloy ingot taken out from the mold is heat-forged together with the heat-retaining metal member. As compared with a method of fitting an alloy ingot into a tube in a related art, the heat-retaining metal member and the alloy ingot can be sufficiently attached to each other and metals can be melted and adhered to each other, and thus it is possible to integrally forge both members with excellent followability. In addition, a stainless steel or a heat-resistant steel which has a smaller deformation resistance than that of the alloy ingot is used as the heat-retaining metal member, and a difference in deformation resistance at a forging temperature between the heat-retaining metal member and the alloy ingot is minimized to be within a predetermined range, thereby preventing the heat-retaining member is only being processed. According to the above-described method, the temperature decrease of the alloy ingot can be more reliably minimized, and thus hot forging can be performed stably and efficiently.
Patent Document 1: JP-A-2001-79633
Patent Document 2: JP-A-S62-3842